It's been a good couple of months to be in mergers and acquisitions on Sand Hill Road. Since April 17, companies Time Warner, WPP, Yahoo, Google and Microsoft have spent over $10.4 billion to acquire online advertising firms.

That sum, the disclosed amount spent on firms DoubleClick, aQuantive, 24/7 Real Media and the Right Media Exchange--does not include the undisclosed amounts spent on a set of relatively smaller acquired companies such asScreenTonic SA, ADTECH AG and Third Screen Media.